1. Field of the Invention
In general, the present invention relates to a method of fabrication of semiconductor processing apparatus from an aluminum substrate. In particular, the invention relates to a method of forming complex shapes which can subsequently be anodized to provide at least one plasma-resistant surface, and particularly a surface which is resistant to halogen-containing plasmas.
2. Brief Description of the Background Art
Semiconductor processing involves a number of different chemical and physical processes whereby minute integrated circuits are created on a substrate. Layers of materials which make up the integrated circuit are created by chemical vapor deposition, physical vapor deposition and epitaxial growth, for example. Some of the layers of material are patterned using photoresist masks and wet and dry etching techniques. Patterns are created within layers by the implantation of dopants at particular locations. The substrate upon which the integrated circuit is created may be silicon, gallium arsenide, glass, or any other appropriate material.
Many of the semiconductor processes used to produce integrated circuits employ halogen or halogen-containing gases or plasmas which are particularly corrosive to processing apparatus surfaces they contact. Some processes use halogen-containing liquids. In addition, since the processes used to create the integrated circuits leave contaminant deposits on the surfaces of the processing apparatus, such deposits are commonly removed using plasma cleaning techniques which employ at least one halogen-containing gas, which is also corrosive.
Aluminum has been widely used as a construction material for semiconductor fabrication equipment, at times because of its conductive properties, and generally because of ease in fabrication and availability at a reasonable price. However, aluminum is susceptible to reaction with halogens such as chlorine, fluorine, and bromine, to produce, for example, AlCl3 (or Al2Cl6); or AlF3; or AlBr3 (or Al2Br6). The aluminum-fluorine compounds can flake off the surfaces of process apparatus parts, causing an eroding away of the parts themselves, and serving as a source of particulate contamination of the process chamber (and parts produced in the chamber). Most of the compounds containing aluminum and chlorine and many of the compounds containing aluminum and bromine are gaseous under semiconductor processing conditions and leave the aluminum structure, creating voids which render the structure unstable and with a surface having questionable integrity.
Porosity of the surface of aluminum semiconductor processing apparatus is of grave concern. We discovered that when a large block of aluminum alloy such as 6061 is machined out (hogged out) to produce a complex shape such as a liner for a processing vessel, the machined surface exhibits a high degree of porosity. We considered extruding a tube, as a technique for obtaining an improved aluminum grain structure, and then joining the tube to a top plate to obtain a process chamber liner shape. However, then the problem shifts to joining of the tube to the top plate. Welding of the tube to the top plate typically creates impurities at the interface of materials coming together in the weld (at the joint of the weld), and the impurities frequently increase porosity at the weld joint. The impurities may be in the form of a filler material used in the welding process or may be in the form of impurities present in the aluminum alloy itself which migrate to the weld joint area during the welding process. Welding is generally defined as a coalescence of metals produced by heating to a suitable temperature with or without the application of pressure, and with or without the use of a filler material. Some of the more commonly used welding techniques include electron-beam welding, laser welding, and solid-phase welding. Solid phase welding processes include, for example, diffusion bonding, friction welding, and ultrasonic joining. Solid phase processes typically produce welds without melting the base material and without the addition of a filler material. Pressure is always employed, and generally some heat is provided. Furnace heating is generally provided in diffusion bonding, while frictional heat is developed in ultrasonic and friction joining.
Welding typically produces stress both at the welding joint and in material adjacent the welding joint. Heat treatment or annealing is commonly used to relieve stress. Aluminum alloys begin to exhibit grain growth at temperatures approaching 345xc2x0 C., which causes precipitation of non-aluminum metals at the grain boundaries. This precipitation may lead to cracking along a weld joint when the weld joint is mechanically loaded, and may lead to cracking along grain boundaries during machining. The precipitation also reduces mechanical properties of the alloy by affecting the uniformity of the alloy composition within the article.
If an aluminum alloy is to perform well in a number of semiconductor process apparatus applications, it should also have desirable mechanical properties. Further, mechanical properties should enable machining to provide an article having the desired final dimensions. For example, if the alloy is too soft, it is difficult to drill a hole, as material tends to stick during the drilling rather than to be removed by the drill. Controlling the dimensions of the machined article is more difficult. There is a penalty in machining cost. In addition, the mechanical properties of the article affect the ability of the article to perform under vacuum, depending on the function of the article. For example, a process chamber must exhibit sufficient structural rigidity and resistance to deformation that it can be properly sealed against high vacuum.
With respect to resistance to a halogen-containing plasma, a preferred means of protection of the aluminum surfaces within a process apparatus has been an anodized aluminum coating. Anodizing is typically an electrolytic oxidation process that produces an integral coating of relatively porous aluminum oxide on an aluminum surface. Despite the use of anodized aluminum protective layers, the lifetime of anodized aluminum parts in semiconductor processing apparatus has been limited, due to the gradual degradation of the protective anodized film. In addition, in the past, the combination of mechanical performance of the article and corrosion resistance of the surface of the article has not been adequately addressed. In attempting to obtain the mechanical properties required for the aluminum alloy body of an article, it is possible to affect the surface of the aluminum alloy in a manner such that the aluminum oxide (anodized) layer does not form a proper interface with the aluminum alloy. This creates a porosity, for example gaps between the aluminum oxide layer and the underlying aluminum surface. We have determined that it is particularly difficult to form a protective anodized coating over a weld joint. The porosity and impurities present at a conventional weld joint interfere with anodization of the aluminum present at the weld joint surface. This porosity promotes a breakdown in the protective aluminum oxide layer, leads to particle formation and a constantly accelerating degradation of the protective aluminum oxide film.
Not only is there significant expense in equipment maintenance and apparatus replacement costs due to degradation of the protective aluminum oxide film, but if a susceptor, for example, develops significant surface defects, these defects can translate through a silicon wafer atop the susceptor, creating device current leakage or even short. The loss of all the devices on a wafer can be at a cost as high as $50,000 to $60,000 or more.
It is readily apparent from the above discussion that there is a long standing need for a method of producing semiconductor apparatus components which have a complex shape (the component is not merely a flat plate, for example), which have adequate mechanical properties for the intended application, and which are protected by an anodized coating which is capable of withstanding a corrosive plasma environment.
We have discovered a method of producing a complex-shaped aluminum alloy article, where welding has been employed to form the article, where an anodized aluminum coating is produced over a surface of the article including a weld joint, and where the anodized aluminum coating provides improved performance over that previously known in the art, when exposed to a corrosive plasma environment.
The welding of elements of the aluminum alloy article to form a complex shape is carried out using frictional welding, or a similar technique which permits welding without the migration of a significant amount of impurities contained in the aluminum alloy toward the weld joint. A significant amount is intended to mean an amount which would significantly harm the subsequent formation of an anodized aluminum oxide protective coating over an aluminum alloy surface including the weld joint. Significant harm refers to the shortening of the performance lifetime of the anodized aluminum article. For example, prior to the present invention, the performance lifetime of an anodized welded aluminum article was shortened by about 80% compared to the lifetime of an anodized non-welded article.
In one embodiment, the particular aluminum alloy which is used to form the body of an article of apparatus may be forged, extruded or rolled, and should have the following composition by weight %: A magnesium content of about 0.1% to about 6.0%, with a mobile impurity atom content of less than 2.0%. Mobile impurity atoms include metal atoms other than magnesium, including transitional metals, semiconductors, and atoms which form semiconductor compounds. Mobile impurity atoms of particular interest include silicon, iron, copper, chromium, titanium and zinc. When the article of apparatus is to be used at operational temperatures which are greater than about 250xc2x0 C., the magnesium content of the aluminum article should range between about 0.1% by weight and about 1.5% by weight of the article and the mobile impurity atom content should be less than about 0.2% by weight.
In a second embodiment, which has provided excellent results, which will be subsequently described in detail, a particular aluminum alloy is used to form the body of a semiconductor apparatus article. The raw aluminum alloy stock may be forged, extruded or rolled. The aluminum alloy should have the following composition (in addition to aluminum) by weight %: a magnesium concentration ranging from about 3.5% to about 4.0%, a silicon concentration ranging from 0% to about 0.03%, an iron concentration ranging from 0% to about 0.03%, a copper concentration ranging from about 0.02% to about 0.07%, a manganese concentration ranging from about 0.005% to about 0.015%, a zinc concentration ranging from about 0.08% to about 0.16%, a chromium concentration ranging from about 0.02% to about 0.07%, and a titanium concentration ranging from 0% to about 0.01%, with other single impurities not exceeding about 0.03% each and other total impurities not exceeding about 0.1%. In some instances, depending on what the impurities are, the other total impurities may be permitted to rise to about 0.2% by weight. In addition, the aluminum alloy is required to meet a particular specification with respect to particulates formed from mobile impurities. Of the particulate agglomerations of impurity compounds, at least 95% of all particles must be less than 5 xcexcm in size. Five (5) % of the particles may range from 5 xcexcm to 20 xcexcm in size. Finally, no more than 0.1% of the particles may be larger than 20 xcexcm, with no particles being larger than 40 xcexcm. This high purity aluminum alloy is referred to as LP(trademark) alloy hereafter. LP(trademark) is a trademark of Applied Materials, Inc. of Santa Clara, Calif.
After welding of the elements of the aluminum alloy article to form a complex-shaped part, the aluminum alloy may optionally be stress relieved at a temperature of about 330xc2x0 C. or less, prior to creation of the aluminum oxide protective film over the article surface. The end use application for the part determines whether stress relief is necessary. A side benefit of the heat treatment process is that it provides additional hardening of the alloy, despite prior art assertions to the contrary with respect to aluminum alloys. It is very important that when heat treatment is used, the heat treatment is carried out using lower peak temperatures than commonly recommended for aluminum alloys. Employment of a peak stress relief temperature of less than about 330xc2x0 C. will ensure the desired material properties of the alloy with respect to grain structure, non-aluminum metal distribution properties, and mechanical properties in the article produced. By controlling the grain size of the aluminum alloy during stress relief, and the distribution of impurities within the alloy, it is possible to avoid or at least significantly reduce the formation of impurities near the surface of an alloy article, which impurities interfere with the formation of an anodized aluminum oxide coating on the surface of the article. This ensures the formation of a uniform anodized coating over the entire article surface, including any weld joints in the article. This method of stress relief works particularly well in combination with the LP(trademark) alloy.
An anodized aluminum oxide protective film is typically applied using an electrolytic oxidation process. Generally, the article to be anodized is immersed as the anode in an acid electrolyte and a DC current is applied. On the surface, the aluminum alloy is electrochemically converted into a layer of aluminum oxide.
Prior to the anodization process, it is important to chemically clean and polish the aluminum alloy surface. The cleaning may be carried out using a method known to those skilled in the art. A particularly effective cleaning may be carried out by contacting the surface of the aluminum article with an acidic solution including about 60% to 90% technical grade phosphoric acid, having a specific gravity of about 1.7 and about 1%-3% by weight of nitric acid. The article temperature during cleaning is typically in the range of about 100xc2x0 C., and the time period the surface of the article is in contact with the cleaning solution typically ranges from about 30 to about 120 seconds. Often, the cleaning process is followed by a deionized water rinse.
The subsequent anodization of the aluminum alloy surface to create an aluminum oxide surface may be carried out using anodization techniques known in the art. A particularly good anodized protective coating may be obtained electrolytically in a water-based solution comprising 10% to 20% by weight sulfuric acid and about 0.5% to 3.0% by weight oxalic acid. The anodizing temperature is typically set within a range from about 5xc2x0 C. to about 25xc2x0 C., and often within a range from about 7xc2x0 C. to about 21xc2x0 C. The article to be xe2x80x9canodizedxe2x80x9d serves as the anode, while an aluminum sheet of standard 6061 serves as the cathode. Generally, during the electrolytic oxidation process the current density, in Amps/Square Foot (ASF) in the electrolytic bath, ranges from about 5 ASF to less than 36 ASF. Further, the xe2x80x9cbarrier layerxe2x80x9d thickness at the base of the aluminum oxide film may be controlled by the operating (anodization) voltage, which typically ranges from about 15 V to about 30 V.